The present invention relates to a power supply circuit attached to a vacuum fluorescent display.
A vacuum fluorescent display is an electron tube which displays a desired pattern by causing electrons emitted from the cathode in the vacuum vessel (envelope) having at least one side which is transparent to impinge on the phosphor applied to the anode and causing the phosphor to emit light. In general, as this vacuum fluorescent display, a display having a triode structure with a grid for controlling the behavior of electrons is most frequently used.
FIG. 15 shows a conventional general vacuum fluorescent display tube and a circuit attached to the vacuum fluorescent display (see Japanese Patent Laid-Open No. 2002-260565 (reference 1)). Referring to FIG. 15, reference numeral 1 denotes a vacuum fluorescent display tube; and 400, a power supply circuit attached to the vacuum fluorescent display tube 1. In the vacuum fluorescent display tube 1, an evacuated envelope 2 incorporates an anode 5 comprised of a plurality of anode electrodes 4 coated with a phosphor 3, a cathode 6 placed to oppose the upper surface of the anode 5, and a grid 7 which is placed between the anode 5 and the cathode 6 to control electrons emitted from the cathode 6. The anode 5 is formed on an anode substrate 8.
In this case, the cathode 6 is a filament coated with an electron emitting material. The cathode 6 is connected to an AC power supply 10 via a center-tapped transformer 9 and is grounded (GND) via the center tap of the transformer 9. With this structure, an AC filament voltage Ef is applied across the cathode 6 (between terminals F1 and F2).
The grid 7 is formed in a mesh pattern and receives a DC voltage VDD2 from a boosting circuit 11. Each anode electrode 4 is connected to a driving circuit 12. The driving circuit 12 also receives the DC voltage VDD2 from the boosting circuit 11. The boosting circuit 11 generates the DC voltage VDD2 for the anode/grid by boosting an input voltage Vin (DC voltage). The driving circuit 12 ON/OFF-controls a positive voltage to be applied to each anode electrode 4 on the basis of input display data.
[Cutoff Voltage]
In a vacuum fluorescent display, when the filament potential drops below the turn-off level of the anode potential, light emission leakage may occur. That is, the filament potential needs to be higher than the turn-off level of the anode potential. This filament potential is called a cutoff voltage.
Referring to FIG. 15, the average voltage (average voltage on filament terminal F1 side) between one terminal of the cathode 6 and the GND is equal to the average voltage (average voltage on filament terminal F2 side) between the other terminal of the cathode 6 and the GND. This average voltage of the cathode 6 is set as a cutoff voltage. This cutoff voltage can be adjusted by the value of a resistor RC1 connected between the GND and the center tap of the transformer 9.
In the above conventional power supply circuit 400, however, since the AC filament voltage Ef is obtained by using the transformer 9, problems (1) to (4) are posed as follows:    (1) producing much noise;    (2) requiring much cost and time for the design of a power supply;    (3) causing flicker when displaying a desired pattern on the vacuum fluorescent display tube 1; and    (4) requiring a large power consumption.